The basic construction of a field device includes a superordinated unit, e.g. as transmitter, which is connected via a cable with a load, e.g. a sensor. The communication of the sensor with the transmitter is controlled by a microcontroller, in general a data processing unit.
The connection of cable to sensor occurs frequently via a plugged connection, for example, a galvanically decoupled, especially an inductive, interface. In this way, electrical signals can be transmitted contactlessly. This galvanic isolation provides advantages as regards corrosion protection, potential isolation, preventing mechanical wear of the plug, etc. The inductive interface is usually embodied as a system having two coils, which are, for example, plugged into one another. Moreover, energy is transmitted via this interface. Only a certain maximum power can be withdrawn from the interface.
Such inductively coupling, plug connection couplings with at least one primary and one secondary winding are sold by the applicant under the “Memosens” mark.
FIG. 1 shows the state of the art. The mentioned sensors with inductive energy- and communication interface utilize for electrical current supply as well as for communication the alternating voltage, which is induced on their coil L1. In order to be able to utilize the alternating voltage for electrical current supply, it is rectified. The positive half-wave of the alternating voltage is rectified via a first one way rectification D1 and then utilized for the positive operating voltage UR2 (schematically on the load R2). IC1 is, in such case, a fixed voltage regulator. The negative half-wave is rectified via a second one way rectification D4 and then utilized both for the negative operating voltage UR3 (schematically on the load R3) and also for the receipt of telegrams UC2. The sending of telegrams is implemented via load modulation (on R4 and C4). The thereto necessary load switch W1 is operated referenced to ground, since it is connected with the circuit ground.
If a sensor is constructed according to FIG. 1, there is, for example, on average 3 mW available at the positive operating voltage UR2, and 1 mW at the negativen operating voltage UR3.
Certain sensor types require no negative operating voltage. Others take so little power from the negative operating voltage that the negative voltage can also be produced via auxiliary circuits, e.g. charge pumps. If these sensors have the above described classical arrangement according to the state of the art, almost the entire available power of the negative operating voltage is not utilized and instead is converted into heat. For example, a power of about 0.8 mW is converted “only” into heat. It is desirable to be able to use this power supplementally as positive supply voltage.